As is well known and understood by those skilled in the art, temperature calibration baths using heat treating salts, or oils, as the calibration medium are being replaced, in many instances, by baths using fluidized solids. As will be appreciated, this use of fluidized solids, such as aluminum oxide, as the calibration medium, served to reduce the possibility of explosion or fire hazard, which were sometime experienced when salts or oils were used. In these regards, the bath was primarily used for thermometer calibration work, and safer to use because the medium is chemically inert, and the danger from spitting, splashing, or spilling type accidents is reduced.
Experience has shown, however, that there exist a series of problems with these fluidized solids temperature calibration baths which both limit their accuracy and their usefulness. Characteristic of such baths is that air flows up through the bottom of the fluidized bed, and fluidizes the solid particles, usually of 100-180 mesh size, with the air then exiting the bath through the top of an open reservoir. With the bath temperature being maintained and controlled by immersion heaters, it has been found that the exiting air carries the aluminum oxide, or other particles along with it ("dusting"), to be deposited on nearby surfaces. Such "dusting" effect presents a potential health hazard for operating personnel, the potential for damage to the test instruments being calibrated, and the possibility of having an adverse effect on the calibration accuracy. Secondly, as the temperature of the bath might run relatively high, e.g., to 1500.degree. F., the exiting air may cause damage to the parts of the instrument located outside the bath--for example, to the heads, to electrical connectors, to dial faces, etc., as well as causing inaccuracies in calibrated readings. With certain temperature sensitive instruments to be calibrated, furthermore, difficulties were encountered in immersing the instrument to the proper depth in the fluidized bath necessary to obtain an accurate indication of bath temperature, for to do so, led to the undesirable intrusion of the fluidized particles into vital parts of the instrument and/or an overheating of vulnerable parts. Temperature sensitive switches, in this regard, were particularly affected by the fluidized particles getting into their working parts. Thus, while, in general, overcoming many of the disadvantages associated with salt and oil type calibration baths, the fluidized solids calibration baths have shown to pose a potential health hazard to temperature calibration bath operators that are exposed to breathing airborne particles during the calibration process, and have an adverse effect on the calibration of temperature measuring test instruments by subjecting them to excessive case temperatures.